Lab 4

Forward Engineering. Using one of the schematics listed below, design a printed-circuit board to build a working copy of the circuit. Assignment:

1. Look up the referenced Application Note and read the description of the circuit
2. Compile a bill of materials and find sources for all the parts (including stock numbers and cost)
3. Enter the schematic into the layout tool
4. Produce the necessary PCB layout files for fabrication
5. (Optional) Simulate the circuit in LTSpice (there are fewer weird ICs here)

The following circuits were all designed by Jim Williams, and the schematics appeared in application notes published by Linear Technology.

• Tachless motor speed controller (App Note 11, page 8)
• Fast response V/F converter (App Note 14, page 4)
• Single-cell V/F converter (App Note 15, page 1)
• Low-distortion sine-wave oscillator (App Note 43, page 33)
• Complete AM radio station (App Note 47, page 52)
• High-speed avalanche pulse generator (App Note 47, page 93)
• Triggered pulse generator (App Note 61, page 21)
• Stabilized sine-wave generator (App Note 98, page 3)
• Output-leveled noise generator (App Note 106, page 11)

In choosing a circuit, be sure that you can find all of the necessary components.  Some parts my be substitutable, some may not.  If you have any questions about part substitutions, please ask. 

For a more complicated project, the following circuits provide additional challenges.

• King Kong V/F converter (App Note 14, page 2).  The output frequency goes up to 100 MHz, and this circuit leverages fast components and requires careful high-speed layout.
• Op amp with 120-volt output stage (App Note 18, page 7). With lethal voltages present, this circuit requires careful layout and construction to prevent circuit damage and engineer injury.  If you choose this circuit, power the circuit from a lower voltage (like plus/minus 48V) for initial testing.
• Son-of-Godzilla amplifier (App Note 21, page 8). This amplifier has a bandwidth over 100 MHz and a tendency to destroy itself. Read the adjustment procedure carefully.
• The Zoo Circuit, a micropower V/F converter (App Note 23, page 11).  The development of this low-power circuit is also described in one of his books.
• 28VDC-to-110VAC sine-wave power converter (App Note 35, page 15). Again, with lethal voltages present, this circuit requires careful layout and construction to prevent circuit damage and engineer injury.  If you choose this circuit, power the circuit from a lower voltage (like plus/minus 48V) for initial testing.

Substitutions:

• Substitute 2N3904 transistor for 2N2369 (for now, try other substitutions later)
• Substitute 1N5711 Schottky diode for HP5082-2810

Op-amp applications

Today's lecture is a review of op-amp applications. There are several good references:

• Walt Jung, Op Amp Applications Handbook, published by Analog Devices.
• Bruce Carter and Thomas R. Brown, Handbook of Operational Amplifier Applications, published by Texas Instruments (originally published by Burr Brown).
• Bob Dobkin and others, AN-31 Op Amp Circuit Collection, originally published by National Semiconductor (there is an "updated" one published by Texas Instruments (who now owns National), but the original National Semiconductor publication is better).

Here is a list of circuits you should know (and other interesting circuits):

• Inverting and noninverting amplifiers
• Summing amplifier (multiple inputs)
• Differencing amplifier (two inputs)
• Instrumentation amplifier (three op amps)
• Integrator
• Noninverting integrator
• Single-pole low-pass filter
• Single-pole high-pass filter
• Differentiators don't work (*)
• Lag and lead transfer functions
• Negative impedance converters and gyrators
• Low-pass and high-pass Sallen-Key filters
• Analog computer block diagrams, which lead to...
• Kerwin-Huelsman-Newcomb (KHN) biquad circuit
• Tow-Thomas biquad circuit
• Full-wave rectifier
• Logarithm and exponential circuits
• Temperature-compensated logarithm and exponential circuits
• Multipliers (log/expo, light-bulb, servomotor, quarter-square, and PWM)

(*) Note that the simple differentiator circuit (input capacitor and feedback resistor) doesn't work because the op-amp feedback loop is unstable (there isn't enough phase margin). The TI Handbook says:

It should be mentioned that of all the circuits presented in this section, the differentiator is the one that will operate least successfully with real components. The capacitive input makes it particularly susceptible to random noise...

The first sentence is correct, but the second sentence is wrong. It's not a "random noise" problem, it's a loop-stability problem. The resistor and the capacitor introduce a low-pass filter in the feedback path, which reduces the phase margin (close to, or even below, zero degrees) of the op-amp loop.

Grr.

Resistors and capacitors

Some notes on specifying passive components. Resistors and capacitors come in a wide variety of options.

For example, resistors types include thick film (resistive ink on ceramic), metal film (nichrome), carbon film, carbon composition, and wirewound. Important specifications and parameters include the power rating (constant power and pulse handling), tolerance, temperature coefficient, and voltage coefficient (nonlinearity). Notable parasitics are the series inductance, the parallel capacitance, thermocouple effects, and excess noise. See references [1] and [2].

Let's look at an example of specifying a resistor part: Panasonic thick-film chip resistors (ERJ series)

• E24 and E96 series values
• Sizes: everything from 01005 to 2512
• Power: 1/32 W to 1 W (based on size)
• Tolerance 0.5% or 1%
• Temperature coefficient: 50 ppm/C to 300 ppm/C
• Operating temperature range: −55C to +125C or +155C

Decoding the part number:

Maximum power rating is a function of package size (and type, tolerance, and temperature).  The power in a resistor is P = V²/R. Thus, the maximum voltage that can be applied across a resistor is a function of its power rating V = sqrt(PR).  For a 01005 resistor (0.031 watts) with value 100 Ω, this limit is only 1.7 volts. For a 2512 resistor (1 watt) with value 1 MΩ, this limit is 1000 volts.  However, the package may have a much lower maximum voltage rating (in this case, the limiting element voltage (LEV) is 200 volts).  The rated continuous working voltage (RCWV) is the lower of the two numbers sqrt(PR) and LEV.

The maximum power rating has to be derated above 70C.

Most Unnecessary Disclaimer Award: "These products generate Joule heat when energized. Carefully position these products so that their heat will not affect the other components."

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Capacitors can be friend or foe. There are many different varieties of capacitors, and they all have their own strengths and weaknesses. See references [1] and [2].  There are a large number of properties that we can use to compare capacitors, including value, voltage, size, self time constant, temperature coefficient, dielectric absorption [3], effective series resistance and, of course, cost.

The choice of dielectric has a big effect on the quality and properties of the capacitor.

• Bypass types: aluminum electrolytic, tantalum electrolytic, high- K ceramic
• Filter types: NPO ceramic, polycarbonate, polyester
• Sample-and-hold types: teflon, mica, polystyrene, polypropylene

In App Note 47 [4], Jim Williams has some warnings about bypass capacitor types (see pages 25 and 26). Figure 60 is downright scary.

Be aware of the effects of voltage and temperature.  Some types of capacitors undergo huge variations over the range of operating voltage and temperature (these plots are from the LT1763 datasheet). Note the huge variation of Y5V types.

Example 1: AVX tantalum capacitors (TPS series, low ESR)

• Values: 0.15 μF to 1500 μF (E6 series, 1.0 1.5 2.2 3.3 4.7 6.8)
• Sizes: 0805 to 2924 (see table of custom case codes)
• Voltage: 2.5 V to 50 V (derate by 2/3 above 85C)
• Tolerance 10% or 20%
• Effective series resistance (ESR): 0.03 Ω to 9 Ω (see chart)
• Operating temperature range: −55C to +125C

Datasheet tour:

• Table of case sizes
• Decoding the part number
• Rated voltage and surge voltage (and derating)
• Table of capacitance, voltage, and ESR
• DCL, DF, ESR, RMS current, and RMS voltage

Example 2: Murata chip monolithic ceramic capacitors (datasheet dated Oct. 1, 2012).  Start with decoding the part number, for example GRM188R71E153KA01D:

• GRM tin-plated layer capacitor
• 18 size 1.6x0.8mm (0603)
• 8 thickness 0.8mm
• R7 temperature X7R type −55C to +125C, change ±15%
• 1E rated voltage 25V
• 153 capacitance 15,000 pF (E6 series)
• K tolerance ±10%
• A01 internal specification code
• D paper tape

Datasheet tour:

• Decoding the part number, pages 15–17
• Actual capacitor listed on page 70
• Temperature characteristics, page 119
• Soldering and mounting, page 129

References:

[1] James Bryant, Walt Jung, and Walt Kester, "Passive components," in Op Amp Applications, Walter G. Jung, Ed. Norwood, Mass.: Analog Devices, 2002, ch. 7-1, pp. 7.1–7.24.

[2] Robert A. Pease, Troubleshooting Analog Circuits. Boston: Butterworth-Heinemann, 1991.

[3] Robert A. Pease, "Understand capacitor soakage to optimize analog systems," EDN, p. 125, Oct. 13, 1982.

[4] Jim Williams, "High speed amplifier techniques: A designer’s companion for wideband circuitry," Linear Technology Corp., Milpitas, Calif., Application Note 47, Aug. 1991.

Lab 3 part 1

The reverse engineering got off to a great start yesterday. Here's some quick photos of the students tearing into their chosen projects. (More updates later.)

Lab 3

Reverse Engineering: Take apart and fully document a piece of commercial electronics (provided). Assemble a complete documentation package describing the circuit, components, and behavior of the object. Deliverables (as a web page):

• Basic test results showing behavior
• Block diagram of system
• Bill of materials of all parts
• Complete schematic of circuit
• Working LTspice or Matlab simulation (matching the test results)

Here are some candidate objects, in rough order of complexity.

Danelectro FAB pedals (D-1 through D-6): distortion, overdrive, metal, echo, chorus, and flange.

Joyo guitar pedals: phaser, tremolo, and octaver (JF-06, JF-09, and JF-12); and Behringer guitar pedals: slow motion, tremolo/pan, and vibrato (SM200, TP300, and UV300).

Korg Monotron variants: Monotron Duo (two VCOs) and Monotron Delay (with echo).

Oscilloscope overdrive

Here's the oscilloscope problem that I was describing in class. If ANY part of the waveform is off the screen, you can't trust the results. Here's an example using a TDS3012B oscilloscope. As we saw in class, the falling edge of the square wave exhibits a little undershoot. At 500 mV/div, the undershoot appears to be about 800 mV (at the bottom of the screen). Note that some of the four-volt square-wave waveform is off the top of the screen.

However, if we move the trace up to the top of the screen, now the undershoot appears to be about 1400 mV (note that the vertical scale is still 500 mV/div).

The only change between these two screenshots is a small rotation of the vertical-position knob. When a large part of the waveform is off the screen, the oscilloscope's vertical amplifier is driven into saturation. When the trace returns to the screen, the increased undershoot is actually the recovery transient of the vertical amplifier. If ANY part of the waveform is off the screen, you can't trust the results.